Localization of earth faults is a challenging task, especially in high impedance earthed networks. There are many factors which deteriorate the accuracy of a calculated fault location estimate, such as fault resistance and load. Distribution networks are especially challenging as they have specific features, which further complicate and challenge fault localization algorithms. These include e.g. non-homogeneity of lines, presence of laterals and load taps.
Impedance based fault location algorithms have become industry standard in modern microprocessor based protection relays. The reason of their popularity is their easy implementation as they utilize the same signals as the other functions. Their performance has proven to be satisfactory in localizing short-circuit faults, but they are often not capable of localizing low current earth faults, i.e. earth faults in high impedance earthed systems. This is due to the fact that an earth fault in high impedance earthed networks differs fundamentally from a short circuit fault. Document: Earth fault distance computation with fundamental frequency signals based on measurements in substation supply bay; Seppo Hänninen, Matti Lehtonen; VTT Research Notes 2153; Espoo 2002, discloses an example of a prior art method for fault localization of single phase earth faults in unearthed, Petersen coil compensated and low-resistance grounded networks. Document EP1304580 discloses a method for calculating the fault point distance to a single-pole earth fault within an electric power network compensated with a Petersen coil.
One important factor affecting the accuracy of impedance based fault localization algorithms is the combined effect of load current and fault resistance. A majority of prior art fault localization algorithms eliminate the load component from measured currents. Typically delta quantities (fault state value minus healthy state value), symmetrical components or a combination of both are used for this. Delta quantities can also be difference values due to e.g. variation of the Petersen coil compensation degree. This has the additional advantage that any systematic measurement errors are eliminated.
Prior art fault localization algorithms are typically based on an assumption that load is tapped to the end point of the electric line (e.g. feeder), i.e. the fault is always assumed to be located in front of the load point. In real medium voltage feeders this assumption is rarely correct. In fact, due to voltage drop considerations, loads are typically located either at the beginning of the feeder or distributed more or less randomly over the entire feeder length. In such cases, the accuracy of prior art fault localization algorithms is deteriorated.
In reality, the power systems are never perfectly balanced in terms of geometry and loading. If the phase-to-earth capacitances of individual phases are not equal, the system produces steady-state zero-sequence quantities. If the loading between phases is not equal, the system produces steady-state negative-sequence quantities. These steady-state sequence quantities are an additional error source for fault locator algorithms. Typically the effect of system steady-state unbalance is removed with use of delta quantities. Based on results obtained from simulations and field tests, this seems not to be a sufficient remedy at least for some algorithms. It is therefore important that the algorithm be designed and tested to be robust and stabile against system unbalance. Moreover, the application of prior art impedance based fault localization algorithms is usually restricted to effectively or low-impedance earthed systems. Therefore such algorithms cannot be applied in high-impedance earthed networks.